4. Simulation of maintenance management systems
The traditional and the MBS maintenance systems were simulated, providing a gauge for
the expected outcomes along with a precursory framework for actual implementation.
The simulated MBS followed the rules previously described. The proposed
management scheme was then implemented and operationally validated at a medical
gown manufacturing plant. The simulated plant has 66 machines, ranging from simple
mechanical sewing machines to state-of-the-art electronic equipments. Their manpower is
comprised of 12 mechanics that attend the machines during the three shifts of operation.
The mechanics are categorised into eight groups, from supervisor to mechanic assistant,
with each group further categorised by hourly pay, minimum years of experience and
education level. The workforce is further classified as mechanical, electrical, and electromechanic.
The machines are grouped in 15 categories, determined by maintenance type
(mechanical, electrical, electro-mechanical), maintenance distribution time, maintenance
complexity levels, and the average energy cost per hour. Standard work time values for
mechanics were fitted to three distributions based on historical data. This methodology is
supported by the existing literature (e.g., Blanchard 1998): the normal distribution, which
generally applies to relatively straightforward maintenance tasks and repairs such as
simple remove and replace tasks; the exponential distribution, which is generally used for
equipment with built-in test capability and a repair scheme of remove and replace; and the
log-normal distribution, which applies to those maintenance tasks and repairs where task
times and frequencies vary.
The machines are assigned to the mechanics according to their specific skills
defined by their respective categories. The assignment also considers the years of
experience and a mechanic’s number of annual hours of formal training within a
category. Some machines require being serviced by a mechanic with not only
mechanical but also electrical knowledge. In such a case, an electro-mechanic is
assigned to such machine. Moreover, the hourly pay of each mechanic is a factor that
affects his machine assignment. Higher paid mechanics are usually assigned to
machines that have a higher level of maintenance complexity in order to justify the
higher labour cost.
The traditional and the proposed MBS assignment policies for the plant were
modelled, and analysed using the ProModelTM simulation software. Both maintenance
management systems consisted of the same numbers of machines and manpower levels,
which are 66 machines and 12 mechanics, respectively. The reliability of every machine was
set to the same level under both systems. Similarly, every mechanic had the same service
time for the machines he could service under both systems. Table 1 shows the run matrix
for this factorial design.
Figure 2 depicts part of the background of the simulation of the traditional system.
It can be seen that every mechanic is dedicated to a fixed number of machines, based on
their assigned category. Figure 3 contrasts the traditional to the MBS management
schemes, wherein it can be observed that each category of mechanics, which is represented
by a group of circles, has multiple assignments that allow the mechanics to service several
machine categories.
Figure 3 further highlights that, under the MBS, maintenance mechanics are given a
number that identifies their ranks in a specific category. It is important to note that the
mechanics are the same as the ones under the traditional system, except that their visual